1. Field of the Invention
In one aspect, the present invention relates to apparatus and methods for subterranean exploration, and to methods of making said apparatus. In another aspect, the present invention relates to sonic or acoustic apparatus and methods for use in well logging operations, and to methods of making said apparatus. In even another aspect, the present invention relates to sonic or acoustic tools and methods including a dipole or monopole acoustic transmitter having broad band frequency characteristics with reduced tool noise for use in oil well logging applications, and to methods of making said tools. In still another aspect, the present invention relates to acoustic down hole tools and methods including a dipole or monopole acoustic transmitter having reduced tool mode noise spawned by the transmitter where the transmitters use an actuator driven piston to create pressure into and out of anti-symmetrically or symmetrically disposed openings in a fluid filled housing, and to methods of making said tools.
2. Description of the Related Art
Sonic or acoustic tools are well known for use in oil well logging applications. Such tools employ both a dipole and monopole transmitter to generate waves used to gather data about the subterranean.
Most dipole and monopole transmitters operate in a pulse mode. Energy is usually stored in a capacitor and when transferred to the transmitter produces an acoustic pulse. Generally, an acoustic pulse shape is largely controlled by physical constants such as component mass, spring constants and transmission path lengths making the pulse shape hard to predict and difficult to control or modify.
Dipole acoustic tools generate and measure flexural waves at the borehole wall. At low frequencies, the flexural wave travels at the same velocity as the shear wave. At higher frequencies, the flexural wave travels at a slower velocity than the shear wave. The time delay or slowness (.DELTA.t) of the flexural wave relative to the shear wave can be computed once the acoustic response is received by a receiver. However, the .DELTA.t computation is difficult due to acoustic waves traveling through the tool sonde, commonly known as tool mode noise. Occasionally, this noise is superimposed upon the flexural wave. When this occurs, it is difficult to filter out the noise or to identify the flexural response.
Unfortunately, the dipole transmitter itself is responsible for generating a majority of the tool mode noise. Sources of this tool noise are transmitters that rely heavily upon their mass to control the pulse shape. For example, commonly utilized transmitters have undesirably high transmitter caused ("spawned") noise that interferes with low frequency flexural waves and subsequent detection of the shear wave.
Of course, transmitters having poor electrical to acoustic efficiency require a large bulky power supply and drive circuitry. With these transmitters, the power supply can account for much of the tool's length. As would be expected, a small diameter acoustic transmitter having high output, high signal-to-noise ratio and the ability to operate in pulse or continuous mode is not available.
Therefore, there is a need in the art for improved well logging methods and sonic tools.
There is another need in the art for sonic methods and tools using an effective dipole or monopole acoustic transmitter for use in oil well logging applications.
There is even another need in the art for sonic methods and tools featuring any of reduced manufacturing costs, rugged construction for hostile environments, or simplified signal processing capabilities, and reduced tool mode noise spawned by the transmitter.
There is still another need in the art for sonic methods and tools having electronically controlled or tuned dipole and monopole transmitters which enhance any of processing, pulse shape or the spectral content.
There is yet another need in the art for sonic methods and tools having dipole and monopole acoustic transmitters with controllable low frequency cutoff values.
These and other needs in the art will become apparent to those of skill in the art upon review of this specification and drawings.